Left: D/H ratio measured in the unlabelled residue. The red dot indicates the polystyrene standard sample value. Solid green line indicates the mean D/H ratio of the residue, dashed black line indicates the VSMOW value, and dashed green line indicates the initial D/H ratio of the icy sample, obtained via the analysis of the N₂ -CH₄ bottle; see text for details. Each blue point in the figure represents the sum of 10 initial acquisition stacks to ensure significant number of counts per stack. Right: D/H ratio measured in the labelled residue. The red dot delineates the estimated initial value based on IR CD₄ and CH₄ bands ratiosusing Eq. (1).